1. Field of the Invention
The present invention relates to an optical module used in a wireless communication system, and more particularly, to an optical module allowing a radio frequency (RF) signal inputted from an external circuit to be delivered to an optical device without any loss in a communication system that employs radio over fiber (ROF) technology delivering an RF signal through an optical fiber.
2. Discussion of Related Art
Recently, as the use of wireless communication such as Internet traffic rapidly increases, various technologies have been developed to wirelessly transmit a wideband signal to a subscriber. For wireless communication between a base station and the subscriber, information should be transported from a central office to the base station without any loss.
In a conventional communication system delivering an RF signal through a line such as a copper line or a coaxial cable, a large number of problems are generated by loss in the line. To solve these problems, ROF technology was developed. Radio-over-Fiber (ROF) link technology employs optical carriers that are intensity modulated by the microwave signals through optical modulators and transmitted or distributed to optical receivers via optical fibers. It takes advantages of excellent characteristics of single mode fiber such as low optical attenuation (0.2 dB/km), wide data bandwidth, and no electromagnetic interference (EMI). According to the ROF technology, an optical module is required to deliver the RF signal to the optical fiber. FIG. 1 is a plan view of a conventional optical module, and FIG. 2 is a cross-sectional view taken along line A1-A2 of FIG. 1.
Referring to FIGS. 1 and 2, an optical device 13 is mounted on a substrate 10 that serves as a ground, and a signal line 12 for delivering an RF signal input from an external circuit 20 to the optical device 13 is formed adjacent to the optical device 13. The signal line 12 is electrically insulated from the substrate 10 by a dielectric 11, and connected with the optical device 13 by a wire or ribbon wire 14.
The signal line 12 is made of a microstrip line, and two open stubs 15 are formed perpendicular to the signal line 12 for impedance matching between the optical device 13 and the external circuit 20.
In order to match the impedance between the external circuit 20 and the optical device 13, the open stubs 15 should be trimmed using a laser while return loss is measured on the signal line 12 in the optical module formed as described above. Specifically, even though the optical module is designed to match the impedance between the external circuit and the optical device, the impedance is probably not matched due to variation in device characteristics, tolerance of the packaging process, and so forth. Hence, at least one of the open stubs 15 should be trimmed in a width or length direction until the impedance is matched. Therefore, after the optical module is fabricated, an additional process is required for impedance matching. In addition, when the variation in device characteristics or the tolerance of the packaging process is serious, the impedance matching cannot be achieved within a desired range so that the fabricated optical module cannot be used. In addition, since the conventional optical module uses stubs to match the impedance, an operating frequency band is difficult to be adjusted. Also, due to a narrow bandwidth characteristic of the stub, the conventional optical module is difficult to be applied to a system requiring a wide bandwidth.
Meanwhile, the conventional optical module is formed such that a bias voltage for driving the optical device is supplied through the signal line 12. Therefore, a bias tee should be additionally installed such that the bias voltage is inputted from the external circuit 20, thereby increasing the size of the optical module. The bias tee comprises a capacitor and an inductor connected in parallel to each other, and thus the RF signal inputted from the external circuit 20 is delivered to the optical device 13 through the capacitor. However, the bias voltage supplied through the inductor cannot be applied to other external circuit 20 due to the capacitor, and the RF signal inputted from the external circuit 20 cannot affect the bias voltage due to the inductor.
FIG. 3 is a graph showing a return loss of the conventional optical module. Line B denotes a return loss measured before trimming the open stubs 15, and line C denotes a return loss measured after trimming the open stubs 15. As shown in FIG. 3, it can be seen that the impedance was matched at a frequency band of 60 GHz and that the impedance could be matched only in a narrow band due to the characteristic of the open stubs 15.